Standard pen ink is the surprise component in a flexible carbon fibre supercapacitor which can be bent in a full circle with barely any loss of performance.

Supercapacitors are devices that can unleash large amounts of charge very quickly and are useful in situations where electricity is required to charge or discharge fast: to balance loads on power grids, for instance.

Just a few millimetres in diameter, the new ink-based supercapacitor outdoes the performance of other carbon fibre-based devices, and can hold up to 10 times more charge.

Researchers from Peking University in Beijing, China, built the device by coating two long thin carbon fibres with the ink, then wrapping them in a flexible plastic casing, filled with electrolyte.

The pen ink was used in the supercapacitor after the same team discovered that it contains carbon nanoparticles - perfect for storing charge. When applied to the carbon electrode it provides an enormous surface area for holding charge: 27 square metres per gram of ink.

Where standard batteries store power via charged ions separated from their chemical partners at the negative electrode, supercapacitors use the flow of current to force electrons directly from one electrode to another, creating an electrical potential which can be harnessed later.